1. Field of the Invention
The present invention relates to a crane control system and, more particularly, to a crane control system fitted for shipping works at ports and the like which can convey suspended parcels to predetermined positions with little swinging motion.
2. Description of the Prior Art
Skilled techniques are needed to operate a crane such as, for example, a container crane in facilities at ports and the like in order to accurately hoist down the suspended parcels to the objective positions while minimizing the swinging motions of the suspended parcels. The experienced crane operators and engineers lack as compared with an increase in materials handling and transportation amounts of cargoes and the automatic control of cranes is more and more required.
Hitherto, as automatic crane control systems in which a crane is driven while the swinging motion of suspended parcels is suppressed, the following systems have been known.
The first system:
The system such that the swing angle of rope by which a parcel is suspended is detected and the rope is controlled in a feedback manner so as to reduce the swinging motion of the parcel is known. However, this system has the problem such that it is hard to put into practical use because of difficulty of detection of the swing angle.
The second system:
For example, as disclosed in JP-A-58-95094 or JP-A-58-95093, the swinging motion of the parcel suspended by the trolley is reduced by changing the velocity of trolley in accordance with a previously calculated objective velocity pattern. According to this system, it is necessary to give an enough tractive force of trolley in order to correct the difference between the actual trolley velocity and a desired velocity which is caused due to the swing of parcel or disturbances by the wind and the like. The trolley cannot be allowed to run to a desired position for the shortest time by making the most of the capability of the driving motor, so that there is the problem such that the cycle time is prolonged.
The third system:
For example, as disclosed in JP-A-55-130480, the acceleration and deceleration of trolley are controlled by only the starting torque when the trolley is started and by only the braking torque when the trolley is stopped, thereby preventing the swing of suspended parcel when the trolley enters the stationary velocity operation or stops at an objective position. In this case, the starting torque upon starting of the trolley is generated in the following manner. Namely, an accelerating operation period is obtained so as to be equalized with a natural period of a pendulum which is determined by the length of rope. An acceleration such that the velocity of trolley becomes an objective velocity (stationary operating velocity) when the trolley was accelerated to a predetermined degree for this accelerating period is obtained. The starting torque is generated so as to obtain this acceleration. When the trolley is stopped, a deceleration to stop the trolley is calculated from the rope length and stationary operating velocity and the braking torque corresponding to this deceleration is determined. Briefly speaking, the third system is realized under the conditions such that when the trolley is started or braked, it can run at a constant acceleration or deceleration without being influenced by the suspended parcel when this parcel is swung and returned. However, if the trolley is started by the actual crane when the suspended parcel is at rest, the inertia of the suspended parcel functions so as to obstruct the acceleration of the trolley until the swing angle of rope becomes the maximum value. Thereafter, when the parcel advances due to the pendulum motion, the parcel acts to facilitate the acceleration of trolley. According to the third system, to prevent the swing of parcel, the trolley needs to be accelerated (or decelerated) at a calculated fixed acceleration (or deceleration) irrespective of the above-mentioned functions of the suspended parcel. However, in the actual crane, it is difficult to realize such a velocity control of the trolley.